The invention relates to the purification of ligands that can be used for the preparation of catalysts.
Metal complex catalysts are a common class of catalysts and are used, for example, in commercial scale hydroformylation, hydrocyanation, olefin isomerization, cyclopropanation, and hydrogenation reactions. The presence of certain metals (primarily transition metals, e.g., iron) in hydroformylation systems is detrimental in that these contaminant metals can promote heavies formation or other side reactions.
The source of contaminant metals, such as iron, in trivalent phosphorous ligands can be from a variety of sources, such as reagents used in the production of the pendant groups during the coupling of phosphorous with the pendant groups, or corrosion of equipment during the production. One source of these metals is impurities in the ligands, such as triphenylphosphine and bisphosphites, supplied to the hydroformylation reactor. Nearly all preparations involve as the last step a coupling reaction of an R-M species with a halogen-phosphorous compound P-X as shown below:n R-M+XnP(R′)x(OR″)y→RnP(R′)x(OR″)y+nMX
where n+x+y=3 (assuming no polydendate moieties). Typically a water wash is used to remove the MX salts (typically M=Na, K, Li or Mg; X=Cl, Br). For example, the RnP(R′)x(OR″)y is dissolved in a non-aqueous solvent such as toluene and the salts are removed by washing with water until the concentration of the dissolved halide, typically chloride, is sufficiently low indicating that the residual MX levels are low. The organic layer is decanted off and the resulting product is recrystallized or distilled as appropriate.
Since the ligands used in the hydroformylation reaction are good at complexing metals, they make it difficult to remove the iron without also removing the expensive catalytic metals, such as rhodium, from the hydroformylation reactor. U.S. Pat. No. 4,143,075 discloses a method for removing iron, but it involves shutting down the commercial production for the treatment.
It would be desirable to have a process for hydroformylation that did not require shutting down the production process to lower the level of iron in the reactor.